The first photos I've ever seen of the Marvel office staff are in MARVEL TALES 1 (Sept 1964), then only about 20 staffers, including secretaries and assistants.
The photos in full size:
page 1
page 2

The next set of photos I saw were in FANTASTIC FOUR ANNUAL 7, Dec 1969, and the staff shown there total 41, and that includes some 1964 staffers who were missing from the first set who still worked for Marvel at that time.




This set I found are the only ones I know of from the intervening 10 years, from a 1975 convention brochure.

The only others I've seen were in issues of FOOM (a Marvel fan-publication) and from Marvel calendars of the late 70's.

Marvel, as I understand it from accounts of it, was an unglamorous crackerbox of an office until in the Shooter years they moved to a more spacious setting. That has taken on a larger-than-life mystique in fandom accounts of what it was like to work there.

Some other places I've seen photos of Marvel staff from the 80's-forward are in issues of EPIC ILLUSTRATED, HULK magazine, BIZARRE ADVENTURES, direct-only titles and MARVEL AGE, COMICS JOURNAL and other fan magazines.

And the blogs I pulled some of these from.